Entomology, chemical ecology, evidence-based environmentalism and science in general. I like big bugs and I cannot lie.

Wednesday, 12 December 2012

What is Chemical Ecology?

One striking thing that I realised from the discussions about Rothamsted’s wheat trial is how little was known about chemical ecology - it’s
very easy to get so absorbed in your own field that you forget that what you’re
working on isn’t common knowledge in the wider world.It’s also a fascinating example of how words
can have very different associations to different people – to me chemical
ecology is a fascinating field of study, but I actually noticed one protestor I
was talking to recoil in horror at the juxtaposition of the friendly, positive
word “ecology” with the word “chemical”, with all its unnatural connotations.

Colloquially, the word “chemical” has come to mean something
artificial, some unpronounceable synthetic substance with unpredictable effects
cooked up in a lab somewhere, but it’s important to realise that in the
technical sense the word chemical simply means a collection of atoms.Using the technical definition every physical
object we encounter is made up of chemicals – water is a chemical, so is the
oxygen we breathe, the vitamins, sugars and proteins that we eat, the keratin
in our hair.(The fact that this field
of study was named something that seems so off-putting to some perhaps just
goes to show that science doesn’t have access to the sort of slick PR machine
many assume it does!).

The science of ecology involves the study of the
relationships between different organisms, and between organisms and their environment.Chemical ecology is a particular subsection
of this discipline, which studies those interactions that are mediated by
chemicals; semiochemicals which convey information, for example smelly
compounds which alert organisms to the presence of suitable or unsuitable food,
mates, or danger, pheromones which allow individual organisms of the same
species to coordinate behaviour (for example the queen mandibular pheromone of
honey bees that prevents workers from laying eggs),
and defensive compounds that species use to wage chemical warfare on one
another – the formic acid wood ants squirt at attackers, the antibiotics
secreted by some fungi to prevent the growth of competitor bacteria on their
food or the
signals used by the parasitic weed Striga to parasitise its host plant
for example.

Queen bee surrounded by workers, image from Wikipedia.

My own research on the smells that attract the fly that transmits trachoma to the tears it feeds on and the faeces it lays its
eggs on, and so the aspect of chemical ecology I’m most familiar with, involves
semiochemicals – the volatile molecules that diffuse through the air and convey
information to the creatures that smell them.These are often oil-soluble chemicals – if you try to think of some of
the strongest smelling things you encounter aromatherapy oils are probably on
the list somewhere – but they can also be things that humans can’t smell, like
carbon dioxide or water.Insects in
general have very acute sense of smell – a bee,
for example can smell the equivalent of a single grain of salt in an
Olympic-sized swimming pool.(This highly
acute sense of smell, incidentally, is why bees are being trained to sniff out
drugs and explosives.)So if you’re
hoping to learn how insects interact with their world, and maybe to control how
they do so, smell is a good place to start.

Sniffer bees in action

There are various ways to find the odours insects can
detect, but one of the most direct is to eavesdrop on what’s going on in their
brains using a technique called electroantennography.A nerve impulse is fundamentally just a spike
of electrical charge, so by very carefully inserting one electrode into the tip
of an insect’s antenna, and another into the area of an insect’s brain
responsible for smell, you can measure how the difference in charge between the
two electrodes varies when the insect is exposed to different smells you think
might be important to it and by doing so find out which ones trigger a nerve
impulse – which ones it’s smelling.From
there you can go on to do laboratory and field tests to find out how the insect
reacts to these smells; does it fly towards them, away from them, or do
something in response to them, like feeding on what smells like tasty food or
laying eggs on what smells like a good place for its young to develop?

Exploiting an insect’s sensory word has one great
advantage over many other pest control methods – as different smells mean
different things to different insects, taking an approach informed by chemical
ecology allows you to target one particular pest species without affecting
others, unlike blanket insecticides for example which may be just as harmful to
beneficial insects or a pest’s natural predators as they are to the pest
itself. Take the coddling moth for example, a pest of apple trees whose caterpillar
is the traditional “worm in the apple”.

Adult females of this species produce a
characteristic pheromone which the male can smell from a great distance away,
and he can then follow the perfume trail to find her and mate.Instead of spraying orchards with insecticide
farmers can now use traps baited with synthetic versions of this pheromone in a
technique called mating disruption – overwhelmed by the strong perfume wafting
from the traps the male can no longer find the female, they both eventually die
alone and frustrated and the apples are protected.

The coddling moth isn’t the only insect species whose
chemical communication can be its downfall.Contrary to popular belief bedbugs don’t actually live in bedlinen, but
spend the day hiding in “refuges”, cracks in walls or furniture.Dozens huddle together in these refuges,
waiting out the day, and find each other using their characteristic smell,
sometimes described as reminiscent of cilantro or coriander (although I won’t
be sprinkling bedbugs on my Thai curry any time soon).They’re not the only ones who can use this
smell though – bedbug infestations mean big business losses for hotels so they
need to be tipped off at the first sign of infection.The most sophisticated bedbug detectors out
there use the smell that bedbugs produce to find them, and then signal that
they’ve done so...by wagging their tails.

That’s right, the best bedbug detectors out there are
dogs.Well, the sniffer dogs need
something to do if the bees are displacing them at airports.

In Kenya a novel farming system exploiting chemicalecology is being pioneered to control stem borer caterpillars.These are the larvae of a number of different
moth species (Chillo partellus, Eldana saccharina, Busseola fusca, Sesmia
calamistis) that basically do exactly what they say on the tin; chomp their way
through the stems of maize plants, boring out the centres, which obviously
doesn’t do a lot of good to either the maize or the farmers who want to eat it.The moths find the maize plant to lay their
eggs on by smell, and that’s where chemical ecology comes in, using a push-pull
strategy of interplanting a plant that smells repellent to the moths with the
maize, to mask its naturally attractive smell, and surrounding the maize crop
with a plant that smells attractive to the moths to lure them away.Cunningly the repellent-smelling intercrop is
a plant called Desmodium, a member of the bean family that enriches the soil
with nitrogen and as a bonus kills the parasitic weed Striga which also reduces
maize yields, and the attractive plant is a grass which can be fed to cattle
and which traps the stemborers with sticky sap.

An understanding of chemical ecology isn’t just
helpful for plant growing either.Kenyan
cattle herders dread nagana, a
disease spread by tsetse flies which causes weightloss and death in their
herds.Attractant traps for tsetse flies
already exist – blue sheets (a colour that the flies find attractive) baited
with carbon dioxide mimicking the exhaled breathe of the animals tsetses feed
on. (Incidentally this is why tsetse flies chase cars: a tsetse’s prey is a
large moving object breathing out carbon dioxide, and a car is a very large,
fast-moving object pumping out large amounts of carbon dioxide).

Tsetse trap, from Wikipedia

But these traps alone aren’t sufficient to protect
the Kenyan cattle herds.The solution
came in the form of repellent collars for the cattle, which mimic the odour of
animal species that tsetse don’t find attractive.These are being rolled out at the momentand, in conjunction with traps, serve as a push-pull system for the tsetse.

How about insects pests that transmit human diseases,
could chemical ecology be used to control them?It’s a possibility.Like tsetse
flies, mosquitoes are attracted to the carbon dioxide in exhaled breath, and
also to various odorous chemicals evaporating from human skin.We may already have something that could
serve as the pull component of a push pull system – a trap baited with carbon
dioxide and a synthetic blend of these chemicals that could be more attractiveto mosquitoes than humans are, and we’re on our way to
develop a push.

At present the only effective wearable mosquito
repellent is DEET, developed by the US military
to protect its soldiers.Although highly
effective it has its drawbacks, sometimes causing irritation or damaging
clothes.Repellents made from lemon eucalyptus look promising but evaporate too quickly from the skin to
be very useful at the moment.The
solution may lie in chemicals that we ourselves produce naturally – it turns
out that as well as the attractive chemicals we all produce some of us also produce natural mosquito repellents.

This is not in fact an oven ready scientist but me in
a survival bag.These are used to
capture the odours that human beings produce, to analyse for chemicals that are
attractive or repellent to mosquitoes, as they’re airtight and have very few
odours of their own.Air that has had
all its own odours purified out with a charcoal filter is blown into the bag,
and the, umm, miasma sucked out and analysed.Maybe someday we’ll be able to use these odours to make everyone smell
utterly repellent, at least to mosquitoes.

Using chemical ecology to study the stimuli useful to
insects gives us a greater understanding of the world from their perspective,
and the more we understand about what chemical information they use to find
resources important to them the more we can manipulate those resources that are
also useful to us, like crop plants or even our own bodies, to reduce the
conflict between us.In an increasingly
resource-constrained world, gaining a better understanding of natural systems
in order to make fewer, more sophisticated changes to better meet our needs is
surely the way to go.

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"It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. . . There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved."— Charles Darwin